The objective of this proposal is to determine the sequential events which occur as myocardial cells undergo the transition from reversible to irreversible injury. The anoxic rat heart has similarities to ischemic injury. We propose to use the in vitro rat heart model to allow precise control of experimental conditions to study the role of mitochondria in cell injury. We wish to study oxygen-induced enzyme release mechanisms and to test new hypotheses developed in previous studies. Methods will include electron microscopy, enzyme histochemistry and functional studies of mitochondria isolated from damaged hearts. We hope to elucidate the significance of heterogenous cellular and mitochondria injury, in predisposing cells to the "oxygen-paradox." The results of the study may provide insights into the mechanisms of reflow injury to ischemic myocardium and to rational development of methods to protect ischemic myocardium.